This invention relates generally to external field activated displays and more particularly concerns an external field activated display sheet which utilizes a medial layer of disperse spheres around which liquid can be moved by applying an external field across the display sheet.
Typically, a display device, in sheet form, comprises a thin sheet, which has many attributes of a paper document. It looks like paper, has ambient light valve behavior like paper (i.e. the brighter the ambient light, the more easily it may be seen), is flexible like paper, can be carried around like paper, can be written on like paper, can be copied like paper, and has nearly the archival memory of paper.
There have been different approaches to making a field induced display sheet such as U.S. Pat. No. 3,668,106 titled xe2x80x9cElectrophoretic Display Devicexe2x80x9d, in which an electrophoretic material in a finely divided powder form is suspended in a suspension medium interposed between a pair of electrodes. When an electric field is imposed across the electrophoretic suspension layer, the spatial distribution of the electrophoretic material in the suspending medium causes the optical reflective property of the suspension layer to change.
In U.S. Pat. No. 3,756,693 titled xe2x80x9cElectrophoretic Display Devicexe2x80x9d, an electrophoretic material in a finely divided powder form is suspended in a suspension medium interposed between a pair of electrodes. A colored layer of a desired pattern is provided at the transparent electrode. When an electric field is imposed between the pair of electrodes, the optical property of the suspension layer changes due to the changing spatial distribution of the electrophoretic material.
An alternate approach was disclosed in U.S. Pat. No. 5,717,283 titled xe2x80x9cDisplay Sheet with a Plurality of Hourglass Shaped Capsules Containing Marking Means Responsive to External Fieldsxe2x80x9d, in which the display sheet contains a plurality of hourglass shaped capsules for each pixel of an image. Each hourglass shaped capsule contains ink in one of its chambers. With the application of an external electric field, ink is moved from one chamber to the other in each hourglass shaped capsule to display an image. Visibility of the ink is otherwise blocked by an opaque medial plane.
A major problem in electrophoretic display sheets is gravitational settling of the electrophoretic pigment particles in directions perpendicular to the field direction. A cure has been segmentation of the display sheet either by encapsulation of the electrophoretic medium into small volumes or introducing a porous matrix. The porous matrix is problematic in that it introduces a high flow impedance and causes trapping of pigment particles against horizontal or concave walls. Smooth spheres have negligible horizontal surfaces and no concavity. Accordingly, it is an object of this invention to provide a hiding layer using an array of spheres, which facilitates transport of pigment particles between sides of a medial plane.
Briefly stated, and in accordance with one aspect of the present invention, an electrophoretic display sheet is disclosed which includes a transparent non-conductive top layer, a non-conductive bottom layer, and a medial layer comprised of a plurality of spheres. The interstices formed by the spheres and the top and bottom sheets are filled with a suspension medium in which is suspended at least one form of electrophoretic material.